It is proposed to continue study of the oxidation of cholesterol, as a representative lipid component of tissue membranes, by oxidants which may occur in polluted air. Chief among these oxidants is singlet molecular oxygen, but ground-site oxygen and the reduced forms of oxygen (peroxide and superoxide) are also being studied, as their relationship to excited-state singlet oxygen is indicated in the literature. It is proposed to determine all the reaction pathways which cholesterol undergoes upon oxidation by these agents as they might occur in tissues exposed to air pollutants, thereby to provide a complete picture of oxidation of an important membrane component cholesterol. With this base of knowledge we shall attempt to examine human and animal tissues which have been exposed to air pollutants for the presence of the established oxidized products, thereby to assess the liklihood that polluted air components are active in the damage of lipid membranes of cells and tissues.